Silicon dioxide is frequently utilized during integrated circuit fabrication. The silicon dioxide may be utilized for its insulative properties. Additionally, or alternatively, the silicon dioxide may be utilized for its etch properties in that it can be a sacrificial material that may be removed selectively relative to other materials, or may be a protective material which is selectively retained while other materials are removed.
In some applications it is desired to remove only a portion of a silicon dioxide layer, while leaving the remainder of the silicon dioxide layer. For instance, it may be desired to strip the upper surface of a silicon dioxide layer to remove contaminants or possible non-homogeneities prior to utilizing the silicon dioxide layer as a gate dielectric.
As another example, it may be desired to form silicon dioxide within a trench to fabricate a trenched isolation region. The silicon dioxide may be initially formed to fill, or even overfill the trench; and it may be desired to remove some of silicon dioxide so that the silicon dioxide ultimately is recessed to beneath an uppermost level of the trench.
A method for removing a portion of a silicon dioxide layer is to utilize a diffusion-limited etch. Such etch will stop after some of the silicon dioxide layer is removed, and before an entirety of the silicon dioxide layer is removed. An example diffusion-limited etch is described with reference to FIGS. 1-3.
FIG. 1 shows a portion of a semiconductor construction 10. The construction includes a semiconductor substrate (or base) 12, and a layer 14 over the substrate.
Substrate 12 may comprise, consist essentially of, or consist of, for example, monocrystalline silicon lightly-doped with background p-type dopant. The terms “semiconductive substrate,” “semiconductor construction” and “semiconductor substrate” mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. Although semiconductor substrate 12 is shown to be homogeneous, in some applications the substrate may comprise one or more electrically insulative materials, electrically conductive materials, and/or semiconductive materials associated with integrated circuit fabrication.
Layer 14 may comprise, consist essentially of, or consist of silicon dioxide.
Layer 14 is exposed to an etchant comprising NHxFy and H*, where “x” and “y” are integers greater than or equal to 1, and H* is an activated form of hydrogen. The activated form of hydrogen may result from, for example, flowing a hydrogen-containing species (for instance, NH3) through a plasma.
The etchant removes silicon dioxide from layer 14, but in the process of such removal, an ammonium fluorosilicate by-product is formed. The ammonium fluorosilicate by-product eventually creates a cap across layer 14. The ammonium fluorosilicate cap is impermeable to the etchant, and accordingly impedes further diffusion of the etchant to layer 14. Such prevents further etching of layer 14. The self-limiting nature of the etch restricts the amount of silicon dioxide removed by the etchant. Accordingly, the etchant removes only a thin portion from across exposed surfaces of the silicon dioxide. In some applications, the etchant may strip less than or equal to about 5 nanometers from exposed surfaces of the silicon dioxide.
FIG. 2 shows construction 10 after removal of some of layer 14, and after formation of the ammonium fluorosilicate (NH4)2SiF6 cap 16 over the remaining portion of layer 14.
In subsequent processing, the ammonium fluorosilicate cap may be removed. Such removal may be accomplished by, for example, heating construction 10 to a temperature above 100° C. to volatilize the ammonium fluorosilicate and/or by utilizing an etch selective for ammonium fluorosilicate relative to the underlying silicon dioxide. FIG. 3 shows construction 10 at a processing stage after removal of the ammonium fluorosilicate cap 16 (FIG. 2).
The technology of FIGS. 1-3 is useful for cleaning and recessing silicon dioxide. However, there are some applications where the technology leads to less than satisfactory results. For instance, the technology is not particularly selective for undoped silicon dioxide relative to doped silicon dioxide (with doped silicon dioxide being, for example, fluorosilicate glass, borophosphosilicate glass, and phosphosilicate glass), and there are applications in which such selectivity would be desired. As another example, the etching technology may be too aggressive during applications in which it is desired to recess silicon dioxide within a trench or other opening. The silicon dioxide within such openings may have keyholes or seams extending therein, and the etch may expand such keyholes and seams to produce undesired results.
It would be desirable to develop improved etch technologies for removing silicon dioxide.